The present invention relates to water-tube boilers such as once-through boilers, natural circulation water-tube boilers and forced circulation water-tube boilers.
The water-tube boiler includes body of which is made up by water tubes. The body arrangement of such a water-tube boilers, for example, that a plurality of water tubes are arranged into an annular shape. In the water-tube boiler of this form, a cylindrical space surrounded by the annular water tube array is used as a combustion chamber. In such a water-tube boiler, heat transfer primarily by radiation is performed within the combustion chamber, and then heat transfer primarily by convection is done in the downstream of the combustion chamber.
In recent years, such water-tube boilers are also desired to be further reduced in NOx and CO. The reduction in NOx, as it stands now, is implemented by fitting low-NOx burners or exhaust-gas re-circulation equipment to the existing boiler bodies. The reduction in CO is implemented by adjusting the state of combustion of the combustion equipment. However, further reduction in NOx and reduction in CO are demanded in keeping up with growing recognitions of environmental issues.
Also, there has been a demand for improvement in boiler efficiency for the purpose of reduction in running cost.
As it stands, such measures as increasing in heat transfer area by providing heat transfer fins in the water tubes, or performing heat recovery from exhaust gas by installing a feed-water preheater. However, for further promotion of energy saving, further improvement in boiler efficiency is demanded.
An object of the invention is to achieve further reduction in NOx and reduction in CO with a simple structure of the boiler body itself and also to achieve further improvement in boiler efficiency.
In order to achieve the above object, the present invention provides a water-tube boiler comprising: a first water tube array made up of a plurality of first water tubes arranged into an annular shape; a combustion chamber defined inside the first water tube array; a first opening defined at part of the first water tube array; a cooling water tube array made up of a plurality of cooling water tubes arranged into an annular shape in a zone within the combustion chamber where burning-reaction ongoing gas is present; gaps provided between adjacent cooling water tubes so as to permit the burning-reaction ongoing gas to flow through; a burning-reaction continuing zone, where burning reaction is continuously effected, provided between the cooling water tube array and the first water tube array; a second water tube array made up of a plurality of second water tubes arranged into an annular shape outside the first water tube array; a second opening defined at part of the second water tube array; and a gas flow passage provided between the first water tube array and the second water tube array, wherein in the gas flow passage, heat transfer area per unit space is larger on the downstream side than on the upstream side.
In an embodiment of the invention, the water-tube boiler is characterized in that in the gas flow passage, heat transfer fins are provided on heat transfer surfaces on the downstream side while the heat transfer fins are not provided on heat transfer surfaces on the upstream side.
In an embodiment of the invention, the water-tube boiler is characterized in that in the gas flow passage, heat transfer fins are provided on at least one of the first water tubes and the second water tubes, and heat transfer area per water tube of the heat transfer fins on the downstream side is larger is than heat transfer area per water tube on the upstream side.
The present invention is embodied as a water-tube boiler of the multiple-tube type. Further, the water-tube boiler of the present invention is applied not only as steam boilers or hot water boilers, but also as heat medium boilers in which a heat medium is heated.
A first water tube array is made up by arranging the plurality of first water tubes into an annular shape, and a combustion chamber is defined inside this first water tube array. A first opening is provided at part of the first water tube array. This first opening may be provided as a single opening having an appropriate width in the circumferential direction, or as a plurality of openings divisionally by interveniently providing one or two first water tubes. A cooling water tube array is made up of a plurality of cooling water tubes arranged into an annular shape, in a zone within the combustion chamber where burning-reaction ongoing gas is present. Gaps are provided between adjacent cooling water tubes so as to permit the burning-reaction ongoing gas to flow through. The burning-reaction ongoing gas includes a flame, being a high-temperature gas under progress of burning reaction. That is, the cooling water tubes are placed within the flame, thus being in contact with the flame. Between the cooling water tube array and the first water tube array, a zone where burning reaction is continuously effected is provided.
Outside the first water tube array, a plurality of second water tubes are arranged in an annular shape, by which a second water tube array is constituted. Between the first water tube array and the second water tube array, is defined a gas flow passage, and this gas flow passage and the combustion chamber communicate with each other via the first opening. A second opening is provided at part of the second water tube array. This second opening may be provided as a single opening or as a plurality of openings, like the first opening. The gas flow passage communicates with the outside of the boiler via the second opening.
Heat transfer area per unit space (so-called heat transfer surface density) in the gas is larger on the downstream side than on the upstream side. For example, in a heat transfer surface in the gas flow passage, i.e., in a heat transfer surface of the first water tube array or the second water tube array on the gas flow passage side, heat transfer fins are provided on the heat transfer surface of the downstream side, while no heat transfer fins are provided on the heat transfer surface of the upstream side. Further, the heat transfer fins are provided on the heat transfer surface of at least one of the first water tubes and the second water tubes on the gas flow passage side, and heat transfer area of the heat transfer fins per water tube is made larger on the downstream side than on the upstream side.
An example of the concrete arrangement for changing the heat transfer area of the heat transfer fins per water tube is given below. The heat transfer fins in the circumferential direction of water tubes is made larger on the downstream side than on the upstream side. Also, the height of the heat transfer fins in a direction vertical to the circumferential surfaces of the water tubes is made larger on the downstream side than on the upstream side. Further, by changing the pitch at which the heat transfer fins are placed, the number of heat transfer fins per water tube is made larger on the downstream side than on the upstream side. These arrangements may be embodied in combination as appropriate.
Flow and reaction of the burning-reaction ongoing gas within the combustion chamber are explained in detail. Burning-reaction ongoing gas that has been generated by the fuel burning in the combustion chamber is cooled by the cooling water tubes, with the temperature lowered, by which the generation of thermal NOx is suppressed. The burning-reaction ongoing gas, which flows through the gaps between the cooling water tubes, contacts the overall surfaces of the cooling water tubes, thus being cooled. As can be explained for Zeldovich mechanism, the higher the temperature of burning reaction, the higher the generation rate of thermal NOx increases considerably; the lower the temperature of burning reaction, the lower the generation rate of thermal NOx, where the generation rate of thermal NOx is considerably lower when the temperature of burning reaction is 1400.degree. C. or lower. Therefore, number and heat transfer area of the cooling water tubes are set in order that the temperature of burning reaction becomes 1400.degree. C. or lower. When the cooling water tube array is made up of a plurality of water tube arrays, the heat transfer area per unit space is increased so that NOx reduction effect by cooling is improved.
The burning-reaction ongoing gas that has passed through the gaps between the cooling water tubes continues burning reaction in a zone between the cooling water tube arrays and the first water tube array, where burning reactions of intermediate products of burning reactions such as CO and HC and unburnt components of the fuel are continuously effected. Since CO remaining in the burning-reaction ongoing gas is oxidized into CO.sub.2, the amount of CO emission from the boiler is reduced.
Within the combustion chamber, radiant heat transfer and convective heat transfer are effected. The gas that has nearly completed the burning reaction flows into the gas flow passage through the first opening, where convective heat transfer is primarily effected in the gas flow passage. The burning-reaction completed gas, after passing through the gas flow passage, is exhausted outside through the second opening.
The burning-reaction ongoing gas flowing through the gas flow passage lowers in temperature as a result of heat exchange with heated fluid within the first water tubes and the second water tubes. Therefore, the burning-reaction completed gas flowing through the gas flow passage decreases in volume increasingly as the gas goes further downstream, with the gas flow rate lowered, resulting in a lowered amount of heat transfer per unit heat transfer area on the downstream side. However, by making the heat transfer area per unit space larger on the downstream side than on the upstream side as described before, the amount of heat transfer on the downstream side is increased, so that the boiler efficiency is improved. Besides, proportionally to the increase in the amount of heat transfer on the down stream side, the amount of heat transfer on the upstream side can be suppressed so that overheating of the water tubes does not occur. Thus, heat loads of the first water tubes and the second water tubes are averagely balanced and the boiler durability is improved.